Most blood chemistry tests require preparation of serum or plasma prior to analysis. To this end, red blood cells and other cellular material are separated from the patient's blood following collection. Typically, blood is collected in evacuated tubes and centrifuged at 2000-3000 rpm for 10-20 minutes.
One type of centrifuge rotor which houses tubes for centrifugation is a fixed angle rotor, in which the tubes are retained in cavities angled relative to the axis of rotation. The dynamics of fixed angle rotors and their ability to enhance the speed of centrifugation are known in the art. The clearing efficiency (K-factor) of fixed angle rotors, which corresponds to the time required to sediment a specific particle in a known medium at a given speed of rotation, can be calculated using the following formula: ##EQU1## where r.sub.1 =radius, in cm, from the outermost point of liquid in the tube to the central axis of rotation, r.sub.2 =radius, in cm, from the center of the top of liquid within the tube to the central axis of rotation and N=rpm.
It is apparent from the above formula that a rotor having tube cavities inclined at a steep angle (approaching 0.degree. in reference to the axis of rotation) can provide the lowest K-factor, and the greatest separation efficiency. However, there are drawbacks associated with using a rotor having steeply angled tube cavities including the fact that the steeper the angle, the greater the tendency of particles to adhere to the outermost wall of the tube, which could lead to contamination of the supernatant.
Another drawback is that the sedimentation boundary formed in a fixed angle rotor centrifuge device is significantly larger than the sedimentation boundary formed in centrifuges using a swing-out style rotor.
Another disadvantage of a steeply angled rotor occurs when gel barrier tubes are used. The position of the gel band along the top side-wall of the processed tube makes it difficult to pipette the supernatant plasma or serum without coming into contact with the gel material. This is especially important in analyzers which employ primary tube sampling capability. Since the thickness of the gel band decreases with the relative steepness of the tube angle, the band can collapse upon deceleration and cause contamination of the supernatant with the particles in the gel.
Still another disadvantage is that there exists a "mixing effect" during reorientation of the tubes from the horizontal position to the vertical position during deceleration, which also increases with the steepness of the tubes within the rotor. During sedimentation, particles travel outward from the axis of rotation until they hit the wall of the tube, then slide downward along the tube wall. This descending layer of increased particle concentration combined with a corresponding ascending layer of reduced concentration fluid creates a fluid flow within the tube which increases the time required to sediment particles, particularly those of low density or irregular shape.
A final disadvantage of using steeply angled tube cavity rotors is that as the steepness of the tube increases, the capacity of the tube decreases. Since the closure of these tubes can trap particles, there is a limit to the tube angle that can be used during centrifugation.
Advances in the speed of test instrumentation have created a demand for faster blood separation methods, and particularly for high speed separation of the blood or serum within the original blood collection tube while maintaining a minimal distortion of the separation boundary within the sample containers.